Sounds like a great FOWLR on the horizon! 2-3 larger fish with a few smaller fish can be done (with really good filtration), but the biggest challenge with a 125 is that your width is probably 18”, right? IMO, 24” gives better room for larger fish to a) turn around, and b) get past another large fish without problems. So with 18” you just want to keep that in mind. Maybe go easy on the amount of rock— and I would definitely put the rock in the middle so there’s two lanes of swim room, and front and back in on the ends as well. Probably a Lion will pick one end of the tank and ‘hover’ there. Some large tank mates might be willing to swim past him in an 18” tank, others may not. If it were me, I might drop a large Puffer and go with a dwarf Puffer or two— like a Valentini and a Blue Spot. They would work with your Lion, Harlequin Tusk, and Tang. (Clown Tank?) A cool Saddleback Clown and maybe a large Wrasse— like a Checkerboard Wrasse.